1. Field of the Invention
The present invention relates to a heat transfer element, more particularly to a heat transfer element for use in a power generating station or a chemical processing plant. Such a heat transfer element can be in the form of a sheet or tube, for example.
2. Background
There are currently over six hundred power generating stations in the European Union. An important feature of these stations is the provision of heat exchangers consisting of a number of radiant panels which serve to transfer heat within the station. There may be around 30,000 square meters of radiant panels in a single heat exchanger. A power generating station may use up to twelve or more heat exchangers.
The radiant panels should not only serve their primary heat transfer function, they should also be robust to withstand the conditions in which they operate. Thus, not only are physical conditions harsh, with hot air and steam at up to about 150° C. flowing at high speed past the panels, but also corrosive chemicals, such as sulphurous and nitrous acids, are present in the air stream. Furthermore, the panels may become clogged with soot or debris, which may also impair their function. The panels are also subjected to rapid thermal cycling.
Conventionally, heat transfer elements used to make the radiant panels have been manufactured from a metal with a vitreous enamel coating. The metal base material, conveniently of mild steel, provides the necessary structural strength to the element and also the required thermal conductivity. A coating of vitreous enamel protects the metal base from the corrosive effects of the surrounding environment.
Recently, attempts have been made to provide heat transfer elements by spraying a metal base with a fluoropolymer. However, the resulting composite element is not economical to manufacture.
In U.S. Pat. No. 4,461,347 there is proposed a heat exchanger assembly comprising coaxially arranged inner and outer pipes. The inner pipe can be formed of high strength metal and ensheathed by an extruded heat shrinkable plastics tube of non-reactive material, such as polytetrafluoroethylene or polypropylene.
A plate heat exchanger comprising at least three plate elements consisting of graphite and a fluoropolymer, such as polyvinylidene fluoride is disclosed in European Patent Specification No. 0 203 213 A1.
British Patent Specification No. 2 255 148A teaches a structurally composite metal and plastics tube in which the metal forms a tubular core having openings throughout its length occupying at least 5% of its total surface area while the plastics material forms imperforate inner and outer layers, each at least 0.1 mm thick, covering the inside and outside of the metal core and integrally joined through the openings.
There is a need to improve upon the performance of heat transfer elements in power generating stations. Thus, it would be desirable to provide a heat transfer element with improved heat transfer properties, with improved anti-fouling properties, with improved resistance to physical and chemical corrosion, and with improved mechanical properties.
All of these desiderata are objects of the present invention.
A further object of the present invention is to provide a heat transfer element with the improved properties referred to above but which is relatively economical to manufacture.